Many industrial processes require large quantities of steam, which is not necessarily required to have a high purity. Nevertheless, in all such processes, environmental issues generally arise in relation to pollutants released in the process itself or in the production of the steam generated. Of particular concern is the release of criteria air contaminants such as nitrogen oxides, sulphur oxides and mercury species as well as carbon dioxide, and the resultant greenhouse gas effect.
Such industrial processes include, but are not limited to, power generation applications such as Rankine cycles, Brayton cycles, combined cycles, gasification processes and cogeneration; and to various general steam applications in the fields of manufacturing or processing of pulp and paper, fertilizers, chemicals and petrochemicals, ethylene, textiles, oil extraction, mining, separation of organic compounds, desalination, provision of district heating, and process use in gas plants and refineries.
For such processes, the production of the necessary quantities of steam may result in the depletion or serious reduction of locally available water supplies, such as rivers or lakes. Furthermore, even if the end use does not require high purity of the steam, the equipment conventionally used for the generation of the steam requires that the input water have a high purity, in particular being substantially free of hydrocarbons and solids, generally resulting in the need for on site water treatment systems.
Conventionally, for many of these processes, boilers provide indirect heat to produce the required steam from treated boiler water. The thermal efficiency of these devices tends to be around 80%; the approximately 20% loss consists of sensible heat and latent heat associated with the dry flue gases and uncondensed moisture exiting through the stack, respectively.
Different equipment is used depending upon the purity of the feedwater and the required steam, and the amount of makeup water required for the process. These include single drum boilers, double drum boilers, once-through steam generators, and direct contact steam generators. Single drum boilers are typical of many utility boilers that use closed cycle steam systems. Fresh water can be relatively easily treated because only a small amount of makeup water must be added to the cycle. Double drum boilers are commonly used for industrial applications because they can more easily separate solids that accumulate as large quantities of process steam are generated using an open cycle. Open cycle operation necessitates very large amounts of expensive feedwater treatment, requiring chemicals and energy. In a double drum arrangement, the lower drum concentrates the solids that can be removed by means of a blowdown extraction. This blowdown results in heat loss from the system, and wastewater release which may require subsequent treatment before release into the environment.
Boilers used in the tar sands currently are an adaptation of the industrial boiler that allows for increasingly poorer feedwater. These once-through steam generators (OTSGs) generally produce 80% quality steam, meaning that there is 20% saturated water included in the product. Since all the product is not steam, the water concentrates the solids formed allowing them to be flushed through the system. In many locations, such as at oil extraction sites or mines, local clean water supplies are being depleted, while new contaminated water sources are created by the wastewater.
A further adaptation to this technology is the use of air-fired direct-contact steam generators. As compared with conventional indirect steam generation, direct contact generators have the advantages of being smaller and more transportable, of having high energy efficiency, the ability to use somewhat lower quality water, and of requiring significantly less capital expense. However, known methods of direct steam generation using air-firing, when compared with conventional steam generation, have the disadvantages that they provide only low quality steam due to dilution by the presence of nitrogen. The nitrogen fraction is generally non-condensable, and the production of carbonic acid from the water and carbon dioxide tends to create the risk of corrosion problems. Additionally, steam generators create air pollution problems, in the form of criteria air contaminants such as nitrogen oxides, sulphur oxides, mercury species, and greenhouse gas emissions which, as noted above, are of steadily increasing concern.
The problems of producing large quantities of steam without environmental damage are particularly significant in the field of oil exploration, separation and extraction processes, which are generally in remote locations. In particular, in oil sands areas such as northern Alberta, Canada, current practices include various processes which require large steam consumption. These processes include, but are not limited to separation of mined tar sand, cyclic steam stimulation (CSS), and steam assisted gravity drainage (SAGD).
For these processes, access to an adequate clean water supply may be difficult, and may create the problem of excessive withdrawal from local sources, resulting in deficiencies of available supply for other uses. Many of these processes result in large quantities of hydrocarbon-contaminated wastewater being deposited above ground, leading to environmental damage which can be severe. Further, the steam generation processes currently in use have associated problems from the release of carbon dioxide.
The above concerns emphasize the need for methods of steam generation for the various processes and uses noted above, and others, without the environmentally damaging discharge of contaminated water, carbon dioxide or other pollutants, and at the same time without excessive depletion of clean water supplies.
In relation to oil field applications, particularly for the production of steam for use specifically in SAGD operations, there are recent publications suggesting that it might be possible to use water which contains contaminants, to avoid unnecessary depletion of clean water supplies or alternatively the need for expensive water treatment facilities.
For example, WO 2009/076763 proposes a “system for low emission hydrocarbon recovery”, in which a compound heat medium for a SAGD process would be generated by combusting a fuel in the presence of an oxidant and a moderator, at elevated temperature and pressure, to create products of combustion, which would be brought into contact with a steam generating medium. The compound heat medium including steam is delivered at pressure with the steam into the intended oil recovery location for use in the recovery process. The publication suggests that some of the carbon dioxide produced will pass into the geological strata or can be recovered separately. The publication suggests that the oxidant can include at least 50% oxygen, but does not address the problems associated with nitrogen which result from the use of air in the process, and overlooks various other problems which would arise from the proposed process.
In particular, the use of 50% oxygen would produce lower quality steam due to the presence of high levels of non-condensable nitrogen. Further, the presence of nitrogen introduces more non-condensable impurities into the produced stream, which would thus significantly decrease the potential to economically create a pure CO2 stream for sequestration purposes.
Still further, the nitrogen presence at high temperatures will increase the thermal NOx formation requiring additional flue gas treatment to minimize this pollutant.
The publication also suggests that low quality water can be used as the steam generating medium, and that accumulated medium can be used as the moderator in the first combustion stage. However, the publication entirely fails to teach a structure which could successfully operate. The suggested structure and arrangement suffers from several serious disadvantages.
In a structure of this nature, ash including liquid slag will be formed in the combustion zone and will run until reaching an environment where lower temperature will create solidification. As the publication only requires that the suggested structure provide combustion temperatures which are sufficient to melt the expected solid contaminants, there is serious risk of solidification in the vessel, and potential blockage downstream of the combustion zone. These blockages will likely occur at the interface between the combustor and the evaporator (steam generation zone) as well as between the evaporator and the slag collection and sump sections (zone) of the system presented due to constrictions in the flow path. In addition, contaminants which may pass from the first combustion stage to the evaporator or slag collection and sump stages can be expected to result in further plugging problems at that stage.
The publication suggests that water collected in the bottom of the vessel can be recirculated into the combustion zone. However, such water would clearly not be suitable for recirculation back into the combustion zone due to the high solid content. The sump and recirculation scheme proposed will concentrate the solids within the moderator (water) stream as shown. If such high concentration solids are re-introduced into the high temperature environment of the combustor, they will inevitably add to potential plugging problems.
The publication suggests that low quality fuels can be used, in order to avoid the high cost of using natural gas or other high quality fuel to generate steam, and identifies the problems of NOx and sulphur compounds which will result from such low quality fuels. If alkali sorbent is used for control of SO2, NOx and other acid products these reactions tend to occur optimally at low temperatures. However, these alkali materials tend to have fouling problems associated with their use. In particular, because of the solid nature of the materials, returning them to the hotter combustor zone will add still further to the slagging and plugging problems identified above.
It is noteworthy that this publication, unlike application U.S. 61/017,828 from which it claims priority, for the first time claims a system operating at “elevated temperature and pressure”. In the priority application, there is no consideration of the role of temperature and pressure in the combustion stage, and no identification of any structure based on these parameters. In the publication WO 2009/076763, the only consideration of them appears to be restricted to the perceived need for higher temperature to melt any solids in the proposed low quality fuels, in that the role of pressure and the selection of appropriate values is not identified.
Thus, although the publication suggests the desirable goal of a system in which lower quality fuels and lower quality water might be used for generation of steam for use in a SAGD process, it does not teach any structure which would in fact be capable of meeting that goal and performing reliably in a continuous operation in the intended environment.
As a further example, CA 2,632,170 proposes an integrated system and method for SAGD heavy oil production using low quality fuel and low quality water. The system includes a two stage process, of combusting the fuel at temperatures and pressures within a selected range, to produce a gaseous flow to a steam generation unit. The fuel is combusted with oxidation gases which can be oxygen, oxygen-enriched air or air; and the combustion gases together with all solids pass through a heat exchanger unit before any solids removal. Any liquids or solids which accumulate in the steam generator are also passed back into the combustor. Thus, an accumulation of unwanted solids can be expected to clog the system, at various locations, in particular at the heat exchanger, and lead to system failure. In particular, it can be expected that the use of low quality fuel and low quality water in the combustor will require careful measures to avoid clogging the system, particularly at regions of constriction, so the addition of still further solids to the first (combustion) stage from the later stages can only be expected to create substantial additional problems.
Similarly to WO 2009/076763 noted above, publication CA 2,632,170 suggests high temperatures and pressures for the combustor stage, but fails to identify a structure in which the proposed parameters can be attained, nor any method of starting up a system which could proceed to operate within the suggested ranges and thereafter continue in effective operation without failure. The use of air in the combustor leads to the problems, identified above in relation to WO 2009/076763.
The publication CA 2,632,170 suggests the use of oxygen, which would, if effective, address some of the problems associated with the nitrogen content of air, but does not teach any actual structure, much less one which is capable of operation within the parameters involved, including the higher temperatures of oxygen firing, and the controls required for safe and effective operation.
The publication fails to show an effective means of solid slag removal from the combustor. In the schematic presented in the publication, all the flows from the combustor are shown as entering a heat exchanger prior to any solids removal. This proposal fails to take into account the serious risk of immediate clogging, if the proposed low quality fuels are used.
Further, the publication suggests that water collected in the bottom of the steam generator be recirculated into the combustion zone. Even if some solids could be removed prior to recirculation, the water would nevertheless have an elevated solid content, rendering it clearly unsuitable for recirculation back into the combustion zone. Any addition of alkali into this stream to address SO2, NOx and other acid products will worsen the situation still further.
Still further, although the publication indicates that low quality water can be used, there is the concurrent requirement for a steady supply of fresh water at the steam generator, at least some of which would appear to be required to be clean water.
Thus the two publications noted above suggest the advantages of systems which could use low quality fuel and low quality water. Although such systems are highly desirable, neither of these publications addresses the problems involved, in that neither teaches any structural system which would or could overcome the problems arising from the varied and wide range of contaminants involved in the low quality input, either for oil recovery operations such as SAGD, to which the publications are directed, much less for any of the broader range of operations to which the present invention is directed.
It has now been found that the use of oxygen-firing, commonly know as oxy-firing, with the associated removal of the problem of the nitrogen dilution of air, in a process for direct steam generation, where combined with suitable steps to deal with contaminants, can allow for the generation of steam for the broad range of operations identified above, including but clearly not limited to oil recovery, with safe and effective separation of carbon dioxide for sequestration or other controlled use or disposal. As discussed further below, it has been found that an apparatus can be provided for such process, in which low quality fuel and water can be used.
The method and apparatus of the invention address and resolve the problems noted above, which remain real and substantial in relation to systems of this nature, and not addressed in practice by the systems proposed in the recent publications directed to steam generation for a SAGD process.
In particular, the apparatus provides for effective removal of contaminants at the earliest feasible stage within the location of the apparatus in which they enter the system, rather than being carried into subsequent locations, with the consequent problems of accumulation and clogging. The apparatus further addresses the problems of ensuring safe start-up and operation, with effective means of ongoing monitoring and cleaning to remove any solids. The controlled temperatures of the combustor, assisted by controlled selective recirculation from the steam generator, maximize the quality of the flue gas passing into the steam generator, and hence minimize the contaminant removal burden within the steam generator, so that any particulate scrubbing means is not overloaded.
The present invention therefore provides a method and apparatus for direct contact steam generation in which the air-firing is replaced by oxygen-firing, and solid combustion fuels can be used. Furthermore, this method and apparatus allows for the substantial reduction of clean water requirements through the use of hydrocarbon-contaminated water in the combustor. This maintains the same advantages which result from air-fired direct contact steam generation, but at the same time eliminates the disadvantages resulting from the nitrogen content of air, that is, low quality steam and non-condensable nitrogen.
The use of oxy-firing in a process for direct steam generation has additional advantages over those of direct air-fired steam generation, including the ability to use wastewater while producing high quality steam, the ability to sequester carbon dioxide, and reduction of equipment size making it more transportable and lower in capital cost. However, to provide the oxygen for the process, there is the additional requirement of the oxygen supply, either by means of an on-site air separation unit, or supply from a remote location in containers such as cylinders.
Oxy-firing of hydrocarbon fuels results in excessive flame temperatures of at least 2830° C. compared with air-firing situations where the flame is typically about 1960° C. The higher temperature from the oxy-firing allows for the use of a broader range of fuels, including solid fuels such as bitumen, asphaltene, coal, and petroleum coke. If conventional combustion equipment is used for oxy-firing, there will generally be a need to quench the flame temperatures to the level of air fired sources; this is typically done by recirculating flue gas. However, in direct fired applications, water can be used to efficiently quench flame temperatures.
In the direct fired steam generation method of the invention, the oxy-fired flame is preferably supplied to the combustor at high pressure. Either pressurized water or recirculated flue gas, or both, is used as necessary to quench the flame temperatures to manageable temperature levels. Water which can be taken from any readily available source, and can be contaminated with dissolved, suspended or entrained solids and hydrocarbons, is sprayed into the system closely downstream of the burner. The flue gas is then transferred to the steam generator where contact with a flow of input water creates the final steam product, and removes solids and pollutants. The features of the method and apparatus allow for the use of input water to the steam generator which is also not clean. In the steam generator, solids can be removed by conventional scrubbing means.
The product steam will consist mostly of carbon dioxide and water vapour. Where the end use for the steam is in a SAGD or CSS process, the complete flow will be pumped underground resulting in sequestration of the carbon dioxide portion. Other processes may require a separation of the carbon dioxide by either pressure let down or some other means. Due to the fact that the combustion products are all converted to the usable product stream the thermal efficiency of the apparatus of the invention is close to 100%.